There is an ever increasing desire for high performance, low cost electronic, optronic, and optoelectronic device structures. Typically, such structures can be prepared by deposition of crystalline thin films onto substrates which act as epitaxial templates for the deposited films. The optimization of such device structures, however, can be limited by the physical properties of the substrate material that is used.
Heretofore, it has been difficult to optimize matching of the coefficient of thermal expansion (CTE) between bonded layers and their substrates because of reasons such as lack of commercially available suitable substrates or prohibitively high cost of production of suitable substrates. For example, Ge and InP substrates are respectively well lattice-matched to films such as GaAs and InGaAs that are used in terrestrial and space-based photovoltaics. The cost of these substrate materials, however, is relatively high compared to competing substrates such as silicon. Similarly, bulk GaN substrates can be preferred for deposition of GaN-based films; however, GaN-based films are conventionally deposited on non-lattice-matched substrates, such as sapphire or SiC, in light of the high cost of bulk GaN substrates. This is not ideal, however, because the high lattice mismatch and high CTE mismatch between GaN films and sapphire or SiC substrates limits the performance of such substrates.
Polycrystalline aluminum nitride substrates can be particularly useful as a relatively low cost substrate in preparation of electronic, optronic, and optoelectronic device structures in light of the useful properties of polycrystalline AlN, such as high strength, oxidation resistance, thermal shock resistance, high thermal conductivity, low electrical conductivity, and resistance to corrosion by liquid metals. Problems still can arise, however, when using polycrystalline AlN substrates in device construction, such as thermal expansion mismatch between the polycrystalline aluminum nitride substrate and other layers bonded thereto. Accordingly, it would be useful to have substrates providing the highly favorable characteristics of polycrystalline AlN while also overcoming the problems associated with CTE mismatch between the substrate and the layers bonded thereto.